A printed circuit board (PCB) that includes printed transmission lines such as microstrip or grounded-coplanar with associated ground planes not only provides compact and lightweight circuits, but also provides broadband frequency capabilities.
However, establishing high quality microwave/mmW connections by means of coaxial connectors becomes difficult at frequencies beyond 20 GHz due to certain fundamental problems manifesting themselves as parasitics at the very area of transition from the planar type of transmission line on the PCB or other substrate to the coaxial type of transmission line, such as a coaxial connector.
In recent years there have been some attempts to improve the planar to coaxial transition. One approach disclosed in U.S. Pat. No. 6,863,548, incorporated herein by this reference, was to add ground vias at the edge of the printed circuit board. Another approach is to add wrap-around plating on the edge of the board where the aforementioned transitions occur.
The former approach to implement a planar to coaxial transition has limited use at higher frequencies due to inherent parasitic inductance of the ground vias connecting the ground planes on the top and bottom of the printed circuit boards as well as the internal ground planes in multilayer printed circuit boards.
The latter approach has a higher frequency capability, but may require manual post-fabrication trimming to remove any metallization that can short the RF trace to wrap-around plating. In addition, such trimming may remove some dielectric material at the very end of the microwave/mmW trace where the transition from planar to coaxial transmission line is being established. This removal of dielectric material may lead to degradation of the performance at frequencies beyond 30 GHz due to parasitic inductance.
There have been some implementations to overcome the problems outlined above by using elongated ground vias at the edge of the printed circuit board. One approach requires one elongated side of such via holes to be edge-plated along the side that coincides with the printed circuit board edge in such manner that when the boards are singulated from the panel, the singulating cut would leave some amount of conductor from the edge-plated elongated via hole to remain on the printed circuit board edge. This may somewhat reduce the parasitic inductance of the via holes, but, due to the fact that elongated vias cannot be drilled, but rather must be routed or machined, the routing diameter of the elongated vias must be substantially greater than the diameter of drilled round vias, such that the remaining portions of the conductor deposited in the elongated vias cannot be placed very close to the microstrip launch area.